High-speed Video Observations of Unusual Dart Leaders in Two Natural Lightning Flashes Striking on Canton Tower

Dart leader behavior preceding eight subsequent return strokes (RS) in two negative cloud-to-ground (CG) lightning flashes striking on the top of Canton tower has been investigated, by using the high-speed video camera. The optical observations show some unusual behavior of dart leader propagation. There appear two branches on the top portion of discharge channel in one flash. Dart leader could propagate downward along either branch, and after RS initiation, there occurs a tiny leader branch near the branching point, following the other branch but normally does not propagate long. Only in one subsequent stoke event, it keeps developing upward so that the entire branched channel can be apparently observed. In the other flash, one dart leader, whose leader tip has the similar characteristics as the normal dart leader, is initiated from the striking point and then develops upward, along the preconditioned channel. The two-dimensional (2-D) average speed of dart leader propagation ranges from 2.83×106 m/s to 1.92×107 m/s, mostly in the order of 106m/s.